When you want to say that something is watertight, that you have no doubt about it —in other words, there is no use in discussing it further —there is a great Italian expression at your disposal. Even if you don't understand why people say it, you can start noticing when people say it and imitate them. And you will soon start sounding like a native as you say it.
Ragazze, la C sta per Catullo e su questo non ci piove.
Girls, the "C" stands for Catullus, and the rain can't touch it [there is no doubt about it].
Captions 71-72, La Ladra EP. 9 L'amico sconosciuto - Part 3Play Caption
It means there is no hole in the argument, but that's not so easy to figure out from the expression, especially since it uses that pesky particle ci that means so many things. It's kind of fun to figure out, or at least imagine why Italians use this colorful expression, and where it comes from.
In Italy, roofs are often made of tiles or tegole. If you move a tegola around, the rain might leak into the house. This can happen accidentally, with high winds, or if someone walks on the roof for some reason, like to clean out the gutters or adjust an antenna. If it rains into the house, ci piove (it rains there, it rains in it).
So besides being a great expression, when talking about leaky roofs, it usually means the rain comes in. It's not easy finding a literal translation that makes sense, which is why we've talked about it here.
When the leak has to do with a pipe or a faucet, we talk about it losing water. We use the verb perdere (to lose, to leak).
Ma... questo non perde più! -No! Non mi dire che l'idraulico s'è degnato? Eva, stamattina qua è passato un vero uomo, eh? Che oltre ad aggiustà [aggiustare] i rubinetti così, proprio tà tà tà l'ha fatto eh!
Well! This no longer leaks! -No! Don't tell me the plumber deigned? Eva, this morning a real man came here, huh? Who besides fixing the faucet just like that, he did it really fast, huh!
Captions 11-14, La Ladra Ep. 1 - Le cose cambiano - Part 3Play Caption
See this lesson about the verb perdere.
Another thing to say when an argument is airtight is: Non fa una piega (there isn't even one wrinkle).
È evidente che avrebbe dovuto vincere Fabiola Alfieri. Allora perché non ha votato per lei? -Perché il direttore di un giornale può essere molto utile alla carriera di un marito come il mio. -Non fa una piega, però non mi convince.
It's clear that Fabiola Alfieri should have won. So why didn't you vote for her? Because the director of a newspaper can be very useful to the career of a husband like mine. That a perfect argument, but it doesn't convince me.
Captions 34-37, Il Commissario Manara S2EP4 - Miss Maremma - Part 4Play Caption
Practice commenting inside your head with su questo non ci piove or non fa una piega when people are justifying, explaining, arguing, debating.
Note that another way to say non fa una piega is non fa una grinza. They both mean the same thing. There's a lesson about this!